pppd troubles

We are having issues and we think that the problem is with pppd. We use
tinit to start a simple serial port mananger program that sets up, monitors
the serial port for incoming calls from a modem and makes outgoing calls via
a modem. This program turns into login and the user logs. The .profile for
the user starts pppd. Every once and a while it seems the serial port gets
stuck with a non existent pid and group and tinit will not start out serial
port manager process. We have tried disconnecting and reconnecting the
modem but tinit doesn’t see this either. This problem is very hard to
recreate, you just have to wait for it to happen again. We are using QNX
4.23A and TCP 4.25C with Socklet and it is all running on a 486. Does
anyone have an idea of what the problem is or how to go about debugging it?

Thanks in advance,
Marc.